steel-pounder wrote on Feb 16^th, 2018 at 11:03am:

If it's okay to ask, what hardness does the core start at and how hard is maximum hardness. I'm asking because I don't suspect that steel with only .2% carbon in it is able harden on quenching, say in the way that a higher carbon knife steel can.

When you mention refined and unrefined, what is supposed to be happening to the core? Just asking, nothing more.

craigd wrote on Feb 16^th, 2018 at 5:49pm:


You are correct 8620 will not get "hard" as will higher carbon steel. It will get to 36 - 40 HRC, which from the information provided here from experianced guys is hard enough.  the carburizing process adds carbon to the skin of the BB though so it will get much Harder. how hard and how thick the "skin" is depends on How efficient the carburizing medium is and how long it is soaked at carburizing temp. 

However! soaking at higher temperatures > 1425f will coursen the grain of the steel which lessens the toughness. by doing a double quench on the carburized BB the grain becomes refined. the core of the Block gets as hard as it is capable of getting, with .2% carbon and the grain is small and tight giving a slightly hard but very tough core. the second hardening at the lower temperature further refines grain at the "skin" and makes it nearly glass hard. 

    Polishing and the basically tempering the block in nitre salt at 550 degrees will leave the block with a beautiful peacock blue color and a hardnes in the mid to low 50's HRC. 
 

To your taste. Either would be adequate.
Chuck

steel-pounder wrote on Feb 17^th, 2018 at 10:13am:

This is only a thought s-p. If you think the steel started off okay, maybe try to avoid the second quench and try an alternative method of thermocycling. I believe that the highest chance of distortion is when quenching, so you may be doubling the chances. My thinking being that you are already starting off with an accurate fit in the action mortice.

Years ago, I tried forging an integral knife blade with what I thought was 52100 round bar. It turned out to be confirmed 8620 when it was clear that it wasn't behaving correctly. Anyway, it's a clunky 2 1/2" blade at the end of a 5" by 3/4" round handle. It's held up great as a cold chisel for chipping cement. It has nothing to do with gun parts, but I think even mistreated 8620 is pretty tough.

And thanks De Bly for the link. I think I found a typo in it, but still good. It says the case will quench to a Rockwell C hardness of 90.

[quote author=55707771405B31405D73661F0 link=1518673102/23#23 date=1518959186]There is no such thing as 90 on the Rockwell C scale. It says that the core will go from 80 Rockwell B to 90 Rb.... [quote]
I didn't realize you had commented again. I was looking under their comments for tempering 8620. It mentions a case with a rockwell c hardness of 90, I suspect just a typo. Maybe they were thinking 60.